Trusses are widely used in engineering and provide a materially efficient way to provide structural strength and an increased ability to bear heavy loads. A multitude of different truss designs exist but all comprise a number of assembled components. While the exact mechanical performance of each type of truss depends on the specific design in question, in general, trusses convert flexing loads into tension and compression loads which are more easily resisted by commonly used truss materials. This allows the truss to support a significantly greater load than would otherwise be possible if the materials were used in a non-truss configuration.
One type of widely used truss design is a Pratt truss (FIG. 1) whose components comprise two parallel chords connected by vertical and diagonal webs. In one example of a Pratt truss design, the parallel chords and the vertical and diagonal webs are hollow metal beams, welded together during assembly. An alternative example is where the components are wooden beams secured to each other during assembly by way of nuts and bolts where each of the beams meet.
Another widely used truss design is a Warren truss (FIG. 2) whose components comprise two parallel chords connected only by diagonal webs. Again, in one example of a Warren truss design, the parallel chords and the diagonal webs are hollow metal beams, welded together during assembly. An alternative example again is where the components are wooden beams secured to each other during assembly by way of nuts and bolts.
As trusses have been widely used in engineering since at least the mid-19th century, it will be appreciated that the skilled person will be well aware of the materials traditionally used to construct a truss as well as a number of traditional ways to connect its components during assembly, particularly in relation to wooden and metal trusses.
One modern development that has started to make its way into civil engineering is the use of lightweight materials that had previously only been used in other industries such as the aerospace and marine industries. One such class of material is Fibre Reinforced Polymer (“FRP”). FRP includes the class of materials known as Glass Reinforced Polymer (GRP). FRP has seen use as a building material in the Startlink Lightweight Building System™ and research into its potential scope and limitations is ongoing (see e.g. Zafari, B 2012, Startlink Building System and Connections for Fibre Reinforced Polymer Structures. Ph.D Thesis, University of Warwick). The use of FRP in construction was further described in a paper titled “The Development of Fibre-Reinforced Polymer (FRP) Composites in Building Construction”, Mark Singleton and John Hutchinson, The second international conference on Sustainable Construction Materials and Technologies, Ancona, Italy, June 2010.
FRP components can be manufactured by way of a pultrusion process which lends itself well to making hollow-section parts which can be used as truss components. Pultrusion consists of coating reinforcing fibres with a resin before pulling them through a heated die in which the composite shape is cured and consolidated.
Despite being a lighter weight material, pultruded FRP is similar in strength to steel in tension and compression but not as stiff. As a truss' strength derives from, amongst other things, its components' ability to deal with tension and compression, FRP is a suitable class of materials from which to construct a truss in order to make it sufficiently stiff without having to use an uneconomical quantity of FRP components.
A skilled person will be aware that stiffness can be imparted by (a) making use of hollow-sections, (b) arranging the hollow-sections into a truss configuration, and (c) pre-stressing the truss to form a gentle camber by putting the upper parts of the truss into compression by applying tension below.
While FRP has seen prior use in truss configurations, such use is confined to embodiments that apply traditional connection and assembly methods that do not make use of FRP's full potential (see e.g. EP0418968B1 and Hizam, R. M. et al (2013) A review of FRP composite truss systems and its connection. In: 22nd Australasian Conference on the Mechanics of Structures and Materials (ACMSM22): Materials to Structures: Advancement through Innovation, 11-14 Dec. 2012, Sydney, Australia).
Much of the know-how relating to assembling, connecting and pre-stressing truss components that applies to metal and wooden materials is not applicable to or not particularly suitable for use with FRP. For example, it will be appreciated by the skilled person that welding FRP components together is not possible. One traditional method that has seen use with FRP is connecting components using steel nuts and bolts. An example of such a method being used with an FRP truss can be seen in Hizam 2013. However, this paper also demonstrates the types of failure that occur in bolted joints at the end of FRP sections. It should also be noted that the bolted sections are open sections like channel or bar which are not as effective as closed hollow sections which have improved buckling resistance, torsional rigidity and stiffness as the paper also demonstrates.
Therefore there is a need for a means to improve the structural integrity of FRP structures including trusses, and for a means of assembling FRP structures in such a way that appropriate levels of tension, compression and stiffness can be imparted and maintained when the structure is put into use.